This invention relates to a method of manufacturing a can by deep drawing a blank cut from a laminate of sheet metal and a polyester film and thereafter wall ironing the cup to a can body. This invention also provides a can made by the method.
Our copending European patent application published number 0312304 describes laminates of linear polyester film and sheet metal such as electrochrome coated steel or aluminium alloy. These laminates are used to manufacture can bodies by a process which includes the steps of cutting a blank from the laminate, applying a lubricant; drawing the blank to a cup; passing the cup through at least one wall ironing die to reduce the side wall thickness and increase its length; trimming the wall ironed side walls to desired height; washing the wall ironed can body to remove lubricant; and drying the can to receive printed decoration.
The laminates, and method described, permit conversion of a circular blank, 140 mm in diameter of laminate (such as aluminium alloy 3004 of thickness 0.315 mm/polyethylene terephthalate 0.015 mm thick) to a wall ironed can 65 mm diameter by 115 mm tall having a side wall thickness of 0.125 mm and a thicker rim margin at the mouth which is 0.188 mm thick.
However, during manufacture of these laminates the metal M (as denoted in FIG. 10) is heated to a temperature T.sub.1 insufficient to melt the entire thickness of polyester film, polymer film A,B is then fed from rolls R.sub.1, R.sub.2 to be applied at pinch rolls P.sub.1, P.sub.2 is the preheated metal. The initial laminate so made is then reheated to a temperature T.sub.2 by an induction heater H.sub.2 before passing through a quenching apparatus which immerses the initial laminate in cold water which travels on the surface of the laminate through ambient air to be collected in a tank T.sub.a from which the quenched laminate is removed. In the process described in EP 0312304 the metal laminate was reheated to a temperature T.sub.2 of about 320.degree. C. This relatively high temperature improves the bond of polyester to aluminium but is expensive to obtain and liable to weaken the aluminium alloy.
We have observed that if the laminating temperature is reduced below 300.degree. C. to retain strength of alloy there is an increased risk of delamination of the polymer film from the side wall of the can body while the can body passes through a conventional spray washing apparatus.
This problem of delamination is also aggravated by more severe wall ironing reductions such as reduction of an alloy 3004 blank 140 mm.times.0.30 mm thick to a side wall 0.105 mm leaving a rim margin thickness of 0.167 mm.
We have also observed that there is a marked increase in delamination if the alloy 3004 blank lacks a pretreatment such as is produced by anodising in, for example, phosphoric acid or conversion to a chromium phosphate.
We have discovered that delamination of the polymer film initiated in the can washing operation can be suppressed by heat treatment before the thermal cycles of drying after washing, and stoving after decorating, which will further improve the bond of film to metal giving an acceptable can.
Summarizing the consequences of delamination:
1. it limits the reduction in wall thickness of the can; PA0 2. it forces one to use lamination temperatures that can reduce the strength of the aluminium alloy so forcing one to use more metal; PA0 3. it adds to the cost by requiring chemical surface treatments to the metal such as alloy 3004. PA0 (a) applying a lubricant to both surfaces of the laminate and cutting a blank from the laminate; PA0 (b) drawing the blank to a cup having a bottom wall and a side wall upstanding from the periphery of the bottom wall; PA0 (c) reducing the thickness of the side wall by pushing the cup through a wall ironing die, and PA0 (d) washing the can body,